By now, knowledge of the space shuttle and its abilities to exit from the earth's atmosphere, enter an orbit of elective duration, perform various functions in space, re-enter the earth's atmosphere, land safely and be reconditioned for succeeding flights into space is widely known. It is also fairly generally known that the outer surface of the space shuttle is covered with heat resistant "tiles", and that from time to time difficulty has been experienced in various of these tiles coming loose from the shuttle surface and being lost. Beyond the foregoing and the media presentations of weightlessness and the spectacular takeoff and landing episodes, very little else is generally known.
In an article entitled "The Toughest Title Job Ever" by Robert L. Dotts, Donald M. Curry and Donald J. Tillian, printed in Chemtech for October 1984, pages 616-626, there is presented a very well written description of the nature and properties of the "Unique thermal protection system (TPS)" that enshrouds the entire outer skin of the space shuttle. As pointed out in the article, the thermal protection system is designed to function for 100 missions with minimal weight gain, maintenance, and refurbishment. The thermal protection system must operate successfully in a variety of environments. The system must be capable of maintaining during ascent and re-entry temperatures of the orbiter's structures below 350.degree. F. The thermal protection system must also withstand launch acoustics, structural deflections and temperatures encountered in deep space as well as earth environments including salt spray, fog, wind, and rain. Different thermal protection materials are applied to different areas of the outer surface of the orbiter depending upon the conditions encountered in the various environments mentioned above. Three of the material systems used to protect the structure of the orbiter are reusable. Two of these systems are formed of a low density silica-ceramic insulation and the third material is a coated nylon felt system. The silica-ceramic tiles are classified as high temperature and low temperature reusable surface insulation, the primary difference between these two being in the nature of the surface coating. The high temperature tiles are coated with a black borosilicate glass, whereas the low temperature tiles are coated with a white borosilicate glass.
The basic silica-ceramic insulation material is manufactured in two densities, one at nine pounds per cubic foot and the other at twenty-two pounds per cubic foot. These materials together cover 70% of the orbiter structure. Most of the area is covered with the lower density material whereas the higher density material is used in areas where a more durable material is necessary. The silica-ceramic material is formed from a short-staple 99.6% pure amorphous silica fibers which are slurried, felted, pressed and sintered to form rigid blocks of insulation material. The individual tiles are then cut from the blocks of insulation material to predetermine size. Ordinarily, the blocks are approximately 6 inches by 6 inches by 1 inch with outer surfaces which are planar. Geometry of the vehicle determines, of course, the shape of other tiles.
To provide a barrier to moisture absorption, the tiles are coated with a borosilicate glass. Those tiles which are adapted to high tempeature surfaces have a coating that contains a black pigment (silicontetraboride).
After coating, the tiles are rendered water repellent to prevent water absorption into the low density insulation. Thereafter the tile remains water repellant until exposed to the high temperatures of reentry.
The tiles are bonded to the outer surface of the orbiter using a polysilicone adhesive and an intervening layer of nylon felt. Although the silica-ceramic tile is a highly effective insulator, it is nevertheless ceramic and possesses low mechanical strength. To prevent damage to the tiles by flexure of the airframe and consequent induction of stresses in the tile, the nylon felt material is used to isolate such strains and prevent damage to the individual tiles. Gaps between the individual tiles are filled with a suitable spacer material.
As pointed out in the aforesaid article, a major technical problem encountered in the flight testing program has been keeping moisture out of the tile. Further details of the moisture absorption characteristics of the orbiter's thermal protection system are founded in the article by Schomburg, Dotts, and Tillian entitled "Moisture Absorption Characteristics of the Orbiter Thermal Protection System and Methods Used to Prevent Water Ingestion", Intersociety Conference on Environmental Systems, July 11-15, 1983.
The present invention has for a primary object the enhancement of the resistance to absorption of water by the silica-ceramic tiles. It has been found that organic silane compositions which have been modified to remove or neutralize substantially completely deleterious impurities in said compositions or the tile to be waterproofed are especially effective to the accomplishment of the foregoing ends.